The present invention relates to a method of color correction in a plate-making process, and more particularly relates to a method of correcting color separation signals obtained from the spot-by-spot photoelectrical scanning of a color original.
To improve the accuracy of color reproductions, as is well known, for the past years there have been employed two alternative methods of color correction; handwork and photographic masking. Particularly, the latter method has been playing a significant part in producing multi-color plates. This method of color correction, however, has such disadvantages that, for instance, many skilled persons are required, its color correction ability is very much limited, its color separation result is not always uniform and further its process is very complicated.
Recently, an electronic color separator, generally called a "color scanner", has been put into practice for color separation as well as color correction to eliminate the disadvantages of the photographic masking method, and now it rather stands in a main current in the relevant technical field. Most of color scanners now in wide use employ a computer of the analog rather than the digital type so as to facilitate speedy computations in color correction. This analog computer, however, is also known to have some disadvantages for all its capability of the speedy computations. For example, it lacks an ability of accomodating itself to a wide range of equations, and therefore operational amplifiers or other components would increase in number. Furthermore, the analog computer is less operative and more susceptible to external factors such as temperature and noise than a digital computer, in addition to its high manufacturing cost.
These disadvantages of the analog computer may be eliminated, at least theoretically, by replacing it with a digital computer. However, as mentioned before, a mere digitalization of the analog type circuit of the computer would cause a tremendous slow-down in the computing speed and would therefore be of no practical use.
Meanwhile, in a recent print-plate making industry, a so-called "direct scanner" has been proposed in order to comply with a demand for more beautiful and better quality of prints as well as to simplify and speed up operations. This direct scanner fulfills two functions, besides that of conventional color correction, of enlarging or reducing an image to a desired size and simultaneously producing a halftone negative or positive -- this process is called a "halftone photography"--. These processes have conventionally been carried out separately by a print-making camera after producing color separation negatives by a color scanner, and color correction by hand-retouching has been given to thus produced color separation negatives. On the other hand, since no step of color correction by hand-retouching is taken in this direct scanner, the color correction of the direct scanner must be performed as accurately as possible.
A recently-developed color correction method may be said to live up to a demand for more accurate color correction to a larger extent. This new method, so to speak, combines merits of both digital and analog computers; that is, it does not only have such merits of the digital as a high reliability, an ability of accomodating itself to a wide variety of color correction computations and high operativeness but it also has such a merit of the analog as an ability of high-speed disposition.
Primarily, the function of a color scanner is to photoelectrically scan a color original spot by spot to obtain color separation signals of each of three primary colors (red, green and blue, which will be referred to as "R", "G" and "B" hereinafter). These R, G and B color separation signals are then fed into a color correction circuit of the color scanner, where eventually calculated are the amounts of printing inks (cyan, magenta, yellow and black) required to reproduce a hue of a certain scanned spot on the color original. Hereinafter, such cyan, magenta, yellow and black inks will be referred to as "C", "M", "Y" and "K".
The above described new method of color correction achieves its aim by taking advantage of the fact that a combination of R.G.B color separation signals representative of a certain spot on the color original exactly corresponds to a combination of the amounts of C, M and Y inks required for reproducing a hue of the spot as a print (in this case, a blank ink signal K is omitted for the purpose of simplifying the descriptions). In other words, the determination of the value of each color separation signal automatically determines the required amount of each ink. According to this color correction method, already-corrected C.M.Y combinations each of which corresponds to a R.G.B combination are previously stored in a memory so that the R.G.B combination obtained by scanning a color original is utilized as an addressing signal for reading out its corresponding C.M.Y combination from the memory.
However, the disadvantage of this method is that, if human eyes can distinguish the density of each of three colors in 200 steps or so, the C.M.Y combinations to be memorized will reach around 200.sup.3 (=8,000,000). In order to read them out at a considerably high speed, a core memory or a semiconductive memory, which is very costly, would have to be installed. Though it is of course feasible to divide each color density to 16 steps (in this case, the total combinations will be 16.sup.3) so as to reduce its memory capacity, each density step becomes too rough and a density difference between the two steps is too big; which results in a quality deterioration of a finished product. A possible way to compensate for the roughness of the steps, namely to prevent a deterioration of a print, is to supplement or interpolate an intermediate value between the two density steps. Even by this way, however, it will turn out to be impossible to reduce the memory capacity to a sufficient level, since not only the C.M.Y signals but also such interpolating signals of intermediate values have to be memorized, and furthermore each of C.M.Y signals needs to be further divided according to its color tone ranging from 0% to 100%.
Another method of reducing a memory capacity of memory to a further extent makes use of the fact that, when each color separation density signal has been digitalized, it is numerically close to the END (equivalent neutral density) value of the color separation density.
Note: the terminology "END" is generally interpreted to imply the visual density which a certain color ink would have if it were converted to a neutral grey, or an achromatic color, by superimposing the just-required amounts of the fundamental colors. In evaluating a patch of yellow ink, for example, it can be expressed in terms of how much density is produced when the proper amount of magenta and cyan is added to produce a neutral. This density is defined as the END value of the yellow ink patch.
According to this method, there is provided a memory for storing such END correction signals each of which corresponds to the difference between the digitalized color separation density value and the END value of that density. This memorized END correction signals are read out from the memory by the digitalized color separation density signals which function as addressing signals, and they are added to the latter signals; whereby suitably corrected signals to be used for recording a color separation negative may be obtained.
A chief advantage of the above method is that absolute values of the END correction signals are so small that not so many bits are reqired to represent them and therefore it becomes possible to effect a considerable reduction of the memory capacity. This point is in fact remarkable, particularly when compared with the prior methods which have had to employ a memory of larger capacity in order to accomodate not only a great number of already-corrected C.M.Y combinations but also interpolating signals for making up such C.M.Y signals.
However, as will be explained in detail, it is still hard to say that this method is the best measure even thinkable for the reduction of the memory capacity, now that it employs a cubic co-ordinate system for a memory as other prior methods do. It is generally known that because of ink contamination or the like a color region actually reproducible by the printing inks are substantially narrowed. For this reason, all what are memorized in the memory are not always used for actual color reproduction. In other words, so long as a cubic co-ordinate system is employed for the memory, it can not be avoided to store even redundant or unnecessary contents in the memory. As another disadvantage of this method, it may be pointed out that jumpings of END correction signals form one density step to another are relatively large in amount.